Add sealed module: modelling domains with sealed types and exhaustive switch

Payment model, permits and the three modifiers, default-branch trap, generic and recursive hierarchies, visitor comparison, run-time enforcement, modules. Transcripts from JDK 21, 25 and 27. Co-Authored-By: Claude Sonnet 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01TF9JWFvJSNm6HVzswzZU5a
2026-09-24 12:03:59 +00:00
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+# sealed — Sealed classes and interfaces
+
+Companion code for the ankurm.com article **Sealed Classes and Interfaces: Modelling Domains with Exhaustive switch**. The explanation lives in the
+article; this folder holds the runnable sources and the transcripts they produced.
+
+```bash
+JDK25=/path/to/jdk-25 JDK27=/path/to/jdk-27 [JDK21=/path/to/jdk-21] ./run.sh    # regenerates output/*.txt
+```
+
+- `src/` compiles and runs unchanged on 21, 25 and 27: the `Payment` model, the three subclass modifiers, generic and recursive hierarchies, the `default` trap and a visitor for comparison.
+- `broken/` fails to compile on purpose (one rule per file).
+- `sep/` shows what the JVM itself enforces when a class file disagrees with the sealed type it claims to extend.
+- `multi/` is a named module, the only place a sealed hierarchy may span packages.
+
+Tested on Temurin 25.0.4.1+1 and 27+35, and OpenJDK 21.0.10 as the baseline.